1. Field of the Invention
The present invention relates generally to a mill for cold-pilgering thick wall metallic tubes to form thin wall tubes and, more particularly, is concerned with an automatic cold-pilger mill stop apparatus for reducing the amount of tube scrap generated during the cold-pilgering operation.
2. Description of the Prior Art
Cold-pilgering is a conventional tube forming operation by which a tube is simultaneously advanced over a stationary mandrel and compressed using two opposing pilger dies resulting in a reduction in the cross-sectional area and in elongation of the tube. Representative of pilgering mills are the ones disclosed and illustrated in Arrington U.S. Pat. No. 3,416,346, Edstrom et al. U.S. Pat. Nos. 3,487,675 and 3,690,850, Naylor et al. U.S. Pat. No. 4,090,386 and Matinlassi U.S. Pat. No. 4,233,834.
Typically, a relatively thick wall tube is reduced and elongated to a thin wall tube by passing through a succession of stations of the cold-pilgering mill with each station being composed of a stationary mandrel and pilger die set. Reduction is effected in both the diameter and wall thickness of the tube by means of the tapered shape of the mandrel and the circumferential tapered shape of grooves formed in the dies which embrace the tube from above and below the mandrel and roll in a constant cycle back and forth along the outer surface of the tube. Between each cycle of die movement, the tube is advanced and rotated incrementally along the mandrel by a feed carriage. The mandrel prevents the tube from collapsing under the force of the pilger dies while at the same time dictates the dimension of the tube inner diameter.
After cold-pilgering, each thin wall tube must be cut to a manageable length, since cold-pilgering greatly increases the length of the tube. The goal is to cut each tube to manageable lengths that will most closely yield a whole number multiple of the length of a finished thin wall tube after final cold-pilgering. However, allowance must also be made for waste (i.e. tube scrap) created by the cold-pilgering process.
One source of waste is the process under which successive thick wall tubes are loaded onto the feedbed of the cold-pilger mill. Cold-pilger mills operate with two thick wall tubes feeding forward at any point in time. The forward or "pushed" tube is the tube being passed over the tapered end of the mandrel and between the pilger dies. The rear tube is the "pusher" tube onto which the feed carriage clamps and advances forward towards the pilger dies. The rear or pusher tube pushes the forward tube between the pilger dies as the feed carriage advances. The mill must be stopped when the forward end of the pusher tube reaches the vicinity of the pilger dies to allow a new pusher tube to be loaded onto the mandrel at the inlet end of the feedbed. The pusher tube now becomes the pushed tube, or the tube to be cold-pilgered. When the dies are stopped to allow the new pusher tube to be loaded onto the mandrel, the stationary pressure exerted by the pilger dies against the portion of the tube positioned between the dies causes an uneven bulge in the tube wall which normally cannot be removed. Consequently, the bulged area must be cut off and becomes waste.
Various methods have heretofore been utilized in an attempt to insure that the reloading shutdown of the cold-pilgering mill occurs when the centerline between the pilger dies is as closely aligned with the facing ends of the pushed and pusher tubes as possible to minimize the length containing bulges that must be cut off. A first method involves the utilization of empirical data to predict and mark when the forward end of the pusher tube reached the centerline of the pilger dies. However, this method has only been reliable to within 18 inches of the abutting, or facing end portions of the pushed and pusher tubes. Another method involves the use of an eddy current sensing system intended to stop the advancement of the pushed tube at the correct time. However, this system has been found to be complex and extremely expensive.
Consequently, a need exists for an apparatus which may be utilized in combination with a cold-pilgering mill to provide an inexpensive, reliable and simple means for ensuring that cold-pilger mill shutdown occurs when the abutting or facing ends of the pushed and pusher tubes are in substantial alignment with the centerline between the pilger dies, regardless of the lengths of the pushed and pusher tubes. Stopping the operation of the cold-pilger mill in this manner will reduce the amount of tube scrap generated and thereby result in an increase in finished tube product yield. Increasing the finished tube product yield obviously results in a substantial material savings and an increase in additional salable tube product.